Here are several ideas. Get back to us if you have further questions
I've used synthetic decking for the frame...being plastic it is rot resistant. The trick is to install some side supports as it is not as stiff as real wood.
the big question is what do you want to grow in the raised beds? corn, tomatoes, peppers, have deeper root structures, lettuce radish etc. is fairly shallow root system. check growing guides for the types of plants you want to grow - and put them in 1 size larger/ deeper. consider your back....elevate them up to at least knee hi if knees or back is bad...and use a garden stool are chair to tend to them.